1. Field of the Invention
The present invention relates to an optical displacement measurement apparatus and an information recording apparatus. More particularly, the present invention can be suitably applied to an encoder that utilizes modulation of an interference light beam owing to diffraction and interference that occur upon the irradiation of light onto an object, and a recording apparatus such as a hard disk that uses the encoder.
2. Related Background Art
As conventional apparatuses for obtaining various physical quantities such as movement, displacement, and the like of an object with high precision by irradiating light onto the object, for example, an optical encoder, a laser Doppler velocimeter, a laser interferometer, and the like are used. The features of these apparatuses using light are high precision and high resolution, and a further size reduction is now required of them for use in a broader application range.
As examples of such apparatuses, a compact encoder that uses three diffraction gratings, as disclosed in Japanese Patent Application Laid-Open No. 62-121314, an apparatus which is obtained by simultaneously realizing a size reduction of such devices with high precision in a simple way, as disclosed in Japanese Patent Application Laid-Open No. 3-279812, and the like are known.
A conventional encoder has high precision, but the following problems remain unsolved.
In the apparatuses disclosed in these references, the optical path of light emitted from a light-emitting source is split into two or more paths, the split light beams are irradiated on a movable diffraction grating, and interference light of diffracted light beams obtained based on these split light beams is received by a photoelectric conversion element. In such an arrangement, if light beams multiplexed by a multiplexing diffraction grating to be brought to interference with each other have a tilt therebetween, fringes are produced on the light-receiving surface of the photoelectric conversion element. In this case, in changes over time in intensity of interference light, the individual interference regions on the light-receiving surface have different phases in the presence of these fringes.
Accordingly, any tilt between the light beams to be multiplexed must be strictly prevented, and the interference states of all the interference regions on the light-receiving surface must be equally adjusted to a constant state. However, it is very difficult to adjust equally the interference states of the interference light beams multiplexed by the diffraction grating to a constant state in all the interference regions. Especially, as the main body resolution is improved, the interference state varies more easily due to any attachment errors, and the like.
Such interference state variations lower the amplitude of a signal detected by averaging changes over time in intensity of the interference light on the light-receiving surface upon production of interference fringes, resulting in an unstable signal amplitude. On the other hand, in an apparatus in which, for example, a multiplexing diffraction grating is made up of a plurality of diffraction gratings with different phases, and interference light components that leave these gratings are individually detected to obtain A- and B-phase signals, if multiplexed light beams have a tilt therebetween, the phase difference between signals obtained from different interference regions is not stable. Also, the amplitudes of signals detected from the individual interference regions are unstable.
Taking the apparatus described in Japanese Patent Application Laid-Open No. 3-279812 as an example, interference state variations owing to attachment errors of a head unit and a scale unit respectively provided with a projection means and a light-receiving means, are problematical. If attachment errors that produce rotation about an axis perpendicular to the formation surface of a diffraction grating have occurred, interference fringes parallel to the arrangement direction of the diffraction grating are formed. On the other hand, if attachment errors that produce rotation about an axis perpendicular to the arrangement direction of the diffraction grating have occurred in the formation surface of the diffraction grating, interference fringes parallel to this rotation axis direction are formed. In the following description, the rotational angle of rotational attachment errors about the axis perpendicular to the surface formed with the diffraction grating will be referred to as an "azimuth angle (.eta.)", and the rotational angle of rotational attachment errors about the axis perpendicular to the arrangement direction of the diffraction grating in the formation surface of the diffraction grating will be referred to as a "rotational angle (.phi.)".
When collimated light beams are used as two light beams to be brought to interference, even when attachment errors in the rotational angle direction have occurred, the two interference light beams have a small angle deviation as long as the errors are small. However, if deviations have occurred extremely in the rotational angle direction, the interference state becomes unstable. On the other hand, when attachment errors in the azimuth angle direction have occurred even slightly, the two interference light beams have a large angle deviation, and the interference state becomes unstable sensitively.